1. Field Of The invention
The present invention is concerned with controlled-torque fastening devices, more particularly with threaded fasteners of the type which can be tightened only to a pre-determined torque level.
2. Description Of The Related Art
Controlled-torque fastening devices are well known in the art. For example, U.S. Pat. No. 3,667,339 discloses a controlled-torque fastening device comprising a threaded shaft fitted with a core member (9 in FIG. 1) about which a shell member (10) is shrink-fitted for frictional engagement therewith. The shell member is designed to be engaged by a driving tool which drives it and the core member in order to drive the fastener. When the driving torque applied to the shell member reaches a certain level, it overcomes frictional engagement between the core and shell members so that the shell member rotates freely about the core member, thereby effectively disengaging the driving tool from driving engagement with the fastener to prevent further tightening of the fastener.
Also well known in the art are controlled-torque threaded fasteners in which a component member of the device is ruptured or permanently deformed at a pre-determined torque level, in order to effectively disengage further application of torque to the fastener. For example, U.S. Pat. No. 3,929,054 discloses, in FIGS. 7-11 thereof, an embodiment of a threaded fastener in which the shaft has a boss 23' which is flared outwardly to retain a nut-like driving plate 19' in camming engagement with a base member 17' affixed to the shaft. The driving plate 19' and the base member 17' have matching serrated surfaces so that driving torque imposed upon the driving plate 19' drives the shaft until the torque reaches a pre-determined level, at which point the base member 17' cams the plate 19' upwardly to permanently deform, i.e., damage or destroy, the boss 23', thereby enabling the plate 19' to rotate freely about the threaded shaft. In FIGS. 1-6 of the patent, there is shown a threaded fastener in which the nut-like plate member 19 is splined to an extension portion of the shaft, the splines 25 fracturing (see FIG. 6) at a pre-determined torque level to effectively disengage the plate member from driving engagement with the shaft and prevent the imposition of further torque on the shaft.
U.S. Pat. No. 3,444,775 discloses a controlled-torque threaded fastener including a tightening nut formed with multiple torque-off collars which are broken off at pre-determined torque levels, thereby permitting staged tightening of bolts to pre-selected torque levels. U.S. Pat. No. 3,267,792 discloses a torque-limited hex nut utilizing a shear pin 34 which fractures when the pre-determined level of shear is reached, thereby allowing the outer portion of the nut to rotate about the inner portion and prevent further tightening. U.S. Pat. Nos. 3,812,757 and 4,029,379 also disclose bolts having a head or neck portion which is fractured at a pre-selected torque level to prevent further tightening.